1. Field of the Invention
The present invention relates to an electrode for lithium secondary battery and lithium secondary battery using this.
2. Related Art
In recent years, development in lithium secondary batteries has been conducted enthusiastically. As for the lithium secondary battery, its battery characteristics such as charge-discharge voltage, charge-discharge cycle life characteristic and storage characteristic are greatly influenced by electrode active materials to be used.
In the electrode active materials capable of lithium storage and release, silicon is variously examined, since silicon can store lithium by being alloyed with lithium so that its theoretical capacity is large. However, since silicon is alloyed with lithium to store, volume expansion and shrinkage during charge-discharge reaction are large. For this reason, the active material is pulverized and separated from a current collector, so that the charge-discharge cycle characteristics are not good and thus silicon is not put into practical use.
In order to solve the above problem, silicon as the active material is attempted to be improved by doping impurity into silicon (Japanese Patent Laid-Open No. 10-199524 (1998)), and by using alloy powder of silicon and a different element (Japanese Patent Laid-Open No. 2000-243389), but sufficient results are not yet obtained.
In addition, there suggests a method of using an intermetallic compound of an element such as silicon and a metal or metalloid as a negative electrode active material so as to improve the cycle characteristic (Japanese Patent Laid-Open No. 10-223221 (1998)). However, a capacity retention rate is improved by making silicon into an intermetallic compound, but the discharge capacity after cycles is not substantially improved. The cause of this is such that since specified stoichiometry exists in an intermetallic compound, a quantity of element for absorbing and discharging lithium in unit volume becomes less, and an initial discharge capacity is reduced more greatly than the case where the element is used in element unit. For example, in the case of an Si3M structure (M: metal or metalloid), a concentration of Si atom for absorbing and discharging lithium is about 75 atomic % of the case of an Si element, and the concentration is reduced to about 67 atomic % in the case of an Si2M structure.
Meanwhile, as an electrode which can solve these problems, the applicants of this invention find an electrode which is formed by depositing a silicon thin film on a current collector according to a thin film forming method such as a CVD method or a sputtering method. It is confirmed that such a kind of the electrode shows high charge-discharge capacity and excellent charge-discharge cycle characteristics.